In advanced lithography machine illumination system, beam shaping elements are used to realize off-axis illumination, which enhances lithography system resolution, enlarges depth of focus, and improves lithography contrast and quality.
Lithography system resolution is the minimum line width that a lithography machine can fabricate on a silicon wafer, it is proportional to light source wavelength. Recently, lithography systems mainly use ultraviolet or deep-ultraviolet laser as the light source (ultraviolet wavelength range: mercury lamp i line wavelength at 365 nm, deep-ultraviolet wavelength range: argon fluoride (ArF) laser wavelength at 193 nm, and krypton fluoride (KrF) laser wavelength at 248 nm. In this invention, we use “ultraviolet” to instead of “ultraviolet and deep-ultraviolet” for simplicity). Short light wavelength is benefit for decreasing critical line dimension and increasing the integration degree of electronic device (such as central process unit (CPU), static memory and etc). Due to the choice of wavelength of light source, the beam shaping element in illumination system is also working at ultraviolet wavelength.
In order to realize exact off-axis illumination mode and high energy efficiency, diffractive optical method is usually adopted for the beam shaping element. Prior to assemble the beam shaping element on the illumination system, its optical performance should be tested. Designed and fabricated beam shaping element requires not only off-axis illumination capability, but also as high energy efficiency as possible by minimizing the energy lose at illumination mode change process. Therefore, the optical performance tests on beam shaping elements comprise far field optical intensity distribution and energy efficiency.
In the existing testing methods, ultraviolet laser, ultraviolet CCD image sensor and ultraviolet energy sensor with the same working as that of beam shaping element, are used to install the testing system (John E Childers, Tom Baker, Tim Emig, James Carriere, Marc D. Himel, Proc. of SPIE Vol. 7430, 74300S, 2009). In the testing system, the output beam of beam shaping element is detected by ultraviolet CCD imaging camera at far field (the focal plane of far field imaging lens), and the output beam is focused on the ultraviolet energy sensor to detect the energy efficiency. The disadvantages of the existing technology includes: the testing system is working at ultraviolet wavelength, which results non-convenient optical path adjustment; the ultraviolet laser, the ultraviolet CCD image sensor and the ultraviolet energy sensor used in testing system are very expensive; the ultraviolet laser uses fluorine-containing mixed gas, which is strongly corrosive and toxic; The ultraviolet light beam cannot be seen by human eyes, which results in difficult optical path alignment and adjustment.